The present invention relates to gelled treatment fluids used during hydrocarbon recovery operations, and more particularly relates, in one embodiment, to methods of xe2x80x9cbreakingxe2x80x9d or reducing the viscosity of treatment fluids containing gelling agents used during hydrocarbon recovery operations.
Hydraulic fracturing is a method of using pump rate and hydraulic pressure to fracture or crack a subterranean formation. Once the crack or cracks are made, high permeability proppant, relative to the formation permeability, is pumped into the fracture to prop open the crack. When the applied pump rates and pressures are reduced or removed from the formation, the crack or fracture cannot close or heal completely because the high permeability proppant keeps the crack open. The propped crack or fracture provides a high permeability path connecting the producing wellbore to a larger formation area to enhance the production of hydrocarbons.
The development of suitable fracturing fluids is a complex art because the fluids must simultaneously meet a number of conditions. For example, they must be stable at high temperatures and/or high pump rates and shear rates that can cause the fluids to degrade and prematurely settle out the proppant before the fracturing operation is complete. Various fluids have been developed, but most commercially used fracturing fluids are aqueous based liquids that have either been gelled or foamed. When the fluids are gelled, typically a polymeric gelling agent, such as a solvatable polysaccharide is used. The thickened or gelled fluid helps keep the proppants within the fluid. Gelling can be accomplished or improved by the use of crosslinking agents or crosslinkers that promote crosslinking of the polymers together, thereby increasing the viscosity of the fluid.
The recovery of fracturing fluids may be accomplished by reducing the viscosity of the fluid to a low value so that it may flow naturally from the formation under the influence of formation fluids. Crosslinked gels generally require viscosity breakers to be injected to reduce the viscosity or xe2x80x9cbreakxe2x80x9d the gel. Enzymes, oxidizers, and acids are known polymer viscosity breakers. Enzymes are effective within a pH range, typically a 2.0 to 10.0 range, with increasing activity as the pH is lowered towards neutral from a pH of 10.0. Most conventional borate crosslinked fracturing fluids and breakers are designed from a fixed high crosslinked fluid pH value at ambient temperature and/or reservoir temperature. Optimizing the pH for a borate crosslinked gel is important to achieve proper crosslinked stability and controlled enzyme breaker activity.
It would be desirable if a viscosity breaking system could be devised to break fracturing fluids gelled with borate crosslinked polymers by reducing the alkaline pH enough to both break the crosslinked gel viscosity and to increase the enzyme breaker activity, if enzymes are present.
Accordingly, it is an object of the present invention to provide a method for breaking the viscosity of aqueous treatment fluids gelled with borate crosslinked polymers used in hydrocarbon recovery operations.
It is another object of the present invention to provide a composition and method for breaking borate crosslinked aqueous fluids by lowering the pH of the alkaline fluid to make better use of an enzyme viscosity breaker.
Still another object of the invention is to provide a method and composition for breaking the viscosity of aqueous fluids gelled with borate crosslinked polymers that can provide better clean up of the crosslinked polymer.
In carrying out these and other objects of the invention, there is provided, in one form, a method for breaking viscosity of aqueous fluids gelled with borate crosslinked materials involving first adding to an aqueous fluid gelled with at least one borate crosslinked polymer a material capable of releasing divalent cations. The material is in an amount effective eventually or ultimately (over time) to reduce the pH of the fluid and remove a portion of borate ion from the crosslinked polymer. Next, the cations complex with at least one pH buffer in the fluid, where the buffer is selected from the group consisting of hydroxides or carbonates. Finally, the pH of the fluid is reduced, and at least a portion of the borate ion is removed from the crosslinked polymer. At least 80% of the divalent cations are slowly released over a period of time up to 48 hours, and where about 20% or less of the divalent cations are released in the first 10 minutes.